Function to add up paths (path tracing rules function)

Anyone know of a function to take a RAM model and an entry and exit point and output a total effect?

i.e., given A model m1 = A<->B; B->C; A->C

pathEffect(m1, from="A", to="C")

Would output the product of paths (a-c)+(a-b * b-c)

If not, it might be a handy addition, and encourage people to report the expected effects in their model rather than just fit. i.e., "All things being equal, this model of our theory suggests that an increase of 50 kg/day of saccharin would decrease lifespan by 1-second, via the following pathways..."